1. Field of the Invention
The present invention generally relates to color scanning systems and more particularly relates to a hybrid illumination system comprising at least a high intensity color-biased light source so as to decrease the exposure time to sensors to increase overall system performance.
2. Description of the Related Art
There are many applications that need optical scanners to convert paper-based objects, such as texts and graphics, to an electronic format that can be subsequently analyzed, distributed and archived. One of the most popular optical scanners is flatbed scanners that convert objects, including pictures and papers, to images that can be used, for example, for building Web pages and optical character recognition. Another emerging optical scanner is what is called sheet-fed scanners that are small and unobtrusive enough to sit between a keyboard and a computer monitor or integrated into a keyboard to provide a handy scanning means. Most optical scanners are referred to as image scanners as the output thereof is generally in digital format.
An image scanner generally includes a sensing module that converts scanning objects optically into electronic images. The sensing module comprises an illumination system, an optical system, an image sensor and an output circuit. The illumination system is used to illuminate an object that is being scanned. The optical system is used to direct and focus the light reflected from the scanning object onto the image sensor. The image sensor comprises a plurality of photodiodes or photocapacitors, referred to as photo-detector hereafter, that are sensitive to light and produce proportional pixel signals accordingly. Therefore corresponding pixel signals are produced in the image sensor when the reflected light is focused thereon and the output circuit is used to convert the pixel signals to an appropriate format to be processed or stored in subsequent systems.
The image sensor is generally in the form of Complementary Metal-Oxide Semiconductor (CMOS) or Charged Couple Device (CCD) and fabricated in either a one-dimensional array or two-dimensional array. The operation of the image sensor often comprises two processes, the first being the light integration process and the second being the readout process. In the light integration process, each photo-detector captures the incident photons of the reflected light and records the total amount of these photons as a charge or pixel signal. After the light integration process the photo-detector is masked so that no further photons are captured and meanwhile the photo-detectors start the readout process during which the pixel signal stored in each photo-detector is individually readout, via a readout passage, to a data bus or video bus.
Many flatbed and sheet-fed scanners use one-dimensional image sensor. This requires either the image sensor or the scanning object to move against each other so as to get the scanning object completely scanned. When the scanners are capable of reproducing colors, there are generally three distinct color components to be generated from the scanning object. Depending on the illumination techniques used and the way the scanning object is scanned, there are several scanning mechanisms that may be found in conventional scanners in the market.
In the first scanning mechanism, the scanner employs an illumination system that comprises multiple light sources, for example, three Light Emitting Diodes (LED), each corresponding to one of three different colors red (R), green (G) and blue (B). The image sensor is normally a single monochrome linear array of photo-detectors. In order to reproduce colors from a scanning object, each line of the object has to be scanned three times. Each time only one LED is turned on. This configuration has several disadvantages. The most noticeable one is the low speed performance because each line of the scanning object has to be scanned three times in sequential order with respect to each turned on LED. The second disadvantage is the uneven light intensity of the light sources. Generally, the intensity of the blue LED is weaker than that of the green LED, and then the light integration period of the blue color needs to be longer than that of the green color. Otherwise the reproduced color in the scanned image may be biased towards green. To improve the scanning speed, the typical practice is to increase the power of each LED so that the light integration period for scanning each color is reduced. However, it is generally expensive to make the high power blue LED.
In the second scanning mechanism, the scanner employs an illumination system that comprises a single light source with white color, typically, a cold cathode fluorescent lamp producing white color. The image sensor is a tri-linear RGB sensor array that can be viewed as a combined three linear arrays of photo-detectors in parallel, each linear array coated with a red filter, a green filter, or a blue filter, respectively. When scanning an object, the illumination system casts the white light that is then reflected, directed and focused on the image sensor. The three linear sensor arrays integrate the light simultaneously and respectively. Because each linear array is coated with a color filter, the image formed with one linear array forms the corresponding color component of the color image. This configuration improves the scanning speed at least three times faster than that in the first configuration, because three color components of a scanning line are formed simultaneously.
This configuration also has several disadvantages. The most noticeable disadvantage is that the cost of the tri-linear RGB sensor array. The manufacturing cost of the tri-linear RGB sensor array can be several magnitudes of that of the single monochrome linear array. Further, it is difficult to align multiple tri-linear RGB sensor arrays for a large width of a scanning object.
Thus there is a great need for a scanning mechanism that provides a generic solution to increase overall performance of a scanning system while not incurring additional cost.